MEMS Microphones are pressure sensitive sensors which must be sensitive to short term small acoustic pressure fluctuations of 1/1000 Pa or even less while remaining insensitive to gradual ambient atmospheric pressure fluctuations. To make a microphone device insensitive to gradual atmospheric pressure fluctuations, a back volume of air which is equilibrated to atmospheric pressure is used on one side of the sensing structure opposite the acoustic port sound inlet. In order to equilibrate the back volume of air to atmospheric pressure, an acoustic leak path is needed to allow air to move between the acoustic inlet and the back volume. This acoustic leak path is normally integrated into the microphone structure around the perimeter of the membrane or through perforations in the membrane.
By way of example, FIG. 1 depicts a portion of a prior art MEMS device 10 which in one embodiment is a microphone. The MEMS microphone 10 includes a membrane 12 which is supported from a backplate 14 by a series of posts 16. Acoustic leak path 18 is provided from a back cavity 20 through slits 22 while acoustic leak path 24 is provided around the perimeter of the membrane 12. The backplate 14 is supported by a substrate 26 through an oxide connecting layer 28.
The presence of the back volume and acoustic leak path creates a high-pass filter which rolls off the microphone sensitivity at low frequencies. This filter is characterized by the corner frequency at which the sensitivity is reduced 3 decibels below the pass-band sensitivity at 1 kHz. This high-pass corner frequency is a key microphone performance parameter which must be maintained and which impacts other microphone performance metrics. For various microphone products, the size of the microphone back volume may change or the corner frequency specification may change, either of which may require an adjustment to the flow resistance of the acoustic leak path to achieve the desired product performance. The current state of the art is to change the size of perforations in the membrane to control the acoustic leak resistance.
The acoustic leak resistance is thus defined by parameters such as the diameter of holes within the membrane, the width and length of slits in the membrane, the gap thickness of air separating the membrane and backplate, and the distance from the membrane edge to the closest perforation in the backplate. One method of precisely controlling the acoustic leak resistance is by using an otherwise sealed membrane having a fully clamped edge combined with one or more precisely defined perforations. The sealed membrane with a clamped edge has the disadvantages of being very stiff and being sensitive to membrane layer stress. More design flexibility can be obtained using a spring supported membrane with perforation slits in the membrane layer defining the springs, but additional length in the slits contributes to reduced acoustic leak resistance and higher corner frequencies.
In view of the foregoing, it would be advantageous to incorporate new features into the MEMS structure to further restrict the acoustic leak path, allowing for both lower corner frequencies and smaller back volumes. It would be advantageous if these acoustic leak control features could be incorporated using known MEMS processes. It would be further advantageous if the leak control features could be easily adapted to provide increased or decreased acoustic leak resistance for particular applications.